In a typical Gas Chromatography (GC) system, a capillary column is connected from the inlet of the GC to a Mass Spectrometer (MS). GC-MS systems are operated in a vacuum outlet condition. When the GC-MS system is operated in this vacuum outlet condition, the optimum pressure at the inlet to the column can be very low. Theoretically, the optimum inlet pressure should be set so low that it would go negative relative to ambient pressure.
Electronic pressure control is common in current GCs. Pressure set-points are entered in gauge pressures (pressure relative to ambient pressure) using the electronic pressure control. Unfortunately, existing electronic pressure controllers in GCs do not allow negative pressure (relative to ambient pressure) set-points. Consequently, it is necessary in existing GC-MS systems to maintain a positive inlet gauge pressure set-point and a resultant positive inlet pressure. Column flow in the capillary column is proportional (i.e., a squared relationship) to the inlet pressure when the GC-MS system is operated in a vacuum outlet condition. As a result, maintaining a positive inlet gauge pressure causes a higher column flow than is actually desired. As shown by the Van Deemter-plot in FIG. 1, the higher column flow, measured as an average linear velocity, above a certain level results in a reduced efficiency (lower HETP=greater efficiency) for mass spectrometry. FIG. 1 shows that for the specified column, conditions, and chemical (carrier gas), an average linear velocity of between approximately 20–30 cm/sec results in the greatest efficiency.